Laser frequency standard employing an optical limiter

ABSTRACT

The disclosed laser has, within its resonator, an active medium and an optical limiter adapted to limit at very low levels in order to provide a very narrow effective linewidth at the peak of the atomic gain-versus-frequency curve. The optical resonator is made sufficiently long, for example, by including therein a subsidiary folded resonator, to have a resonance within the narrow effective linewidth. The limiter provides the laser with a self-stabilizing characteristic that minimizes the effects of random fluctuations in the cavity loss or in the gain of the active medium.

United States Patent Inventor Peter W. Smith Little Silver, N.J. App].No. 713,923 Filed Mar. 18, I968 Patented Apr. 20, 1971 Assignee BellTelephone Laboratories, Incorporated Murray Hill, Berkeley Heights, NJ.

LASER FREQUENCY STANDARD EMPLOYING AN OPTICAL LIMITER 4 Claims, 1Drawing Fig.

U.S. CI 331/945, 356/256 Int. Cl. H0ls 3/10 FieldofSearch 33I/94.5

References Cited UNITED STATES PATENTS 3,434,779 3/1969 Damen et a1331/945 LENS LIQUID THERMAL OTHER REFERENCES I-Ierriott et a]. FoldedOptical Delay Lines Applied Optics, Vol. 4, No. 8, Aug. I965, pp 883-889 Primary ExaminerRonald L. Wibert Assistant Examiner-R. .I. WebsterAttorneys-R. J. Guenther and Arthur J. Torsiglieri ABSTRACT: Thedisclosed laser has, within its resonator, an active medium and anoptical limiter adapted to limit at very low levels in order to providea very narrow effective Iinewidth at the peak of the atomicgain-versus-frequency curve. The optical resonator is made sufficientlylong, for example, by including therein a subsidiary folded resonator,to have a resonance within the narrow effective linewidth. The limiterprovides the laser with a self-stabilizing characteristic that minimizesthe effects of random fluctuations in the cavity loss or in the gain ofthe active medium.

OPTICAL DELA LINE 20 OUT PATENTED M20197:

lNl/ENTOR R W. SM/ TH w. A TTORNEV LASER FREQUENCY STANDARD EMPLOYING ANOPTICAL LIMITER BACKGROUND OF THE INVENTION This invention relates tolasers having relatively narrow effective linewidths. A laser with asufiiciently narrow linewidth at the peak of the atomicgain-versus-frequency curve is very useful as a frequency standard.

Although it has long been hoped that lasers might be useful as frequencystandards, various effects exist which broaden the linewidth of a laser,that is, the band of frequencies at which stimulated radiation can beemitted from a laser under appropriate tuning conditions. Moreover,various disturbances inherent in the operationof lasers tend to move theoscillation frequency or frequencies around within this linewidth.Typically, mechanical fluctuations in the length of the laser cavitycause the frequency of laser oscillation to vary even though-oscillationis only occurring at a single resonator mode.

Various techniques have been proposed to counteract thefrequency-varying effect of fluctuations in the laser cavity length. Forexample, it has been proposed that a frequency standard could be builtconsisting of a chain of laser amplifiers separated byfrequency-independent constant losses. The output radiation will have amuch narrower bandwidth than the atomic gain curve if the losses arelarge enough so that the gain of the amplifiers is everywhereessentially unsaturated. The output consists of spontaneous emissionamplified by passage through the system. If the gain is unsaturated, thegain-versus-frequency characteristic is peaked at the atomic line centerand the radiation is progressively narrowed in frequency as it passesthrough the system. In practice, extremely long length of gain mediumand very close control of losses is required for this arrangement.

As a consequence of the foregoing difficulties, most atomic resonancefrequency standards do not employ lasers or multiple-pass opticalstimulated emission arrangements.

Very few frequency standards have been proposed to operate at opticalfrequencies. However, some have been proposed to operate by repetitivelydoubling lower (millimeter wave) frequencies.

It would be desirable to have frequency standards operating directly atoptical frequencies, inasmuch as this would reduce problems of frequencydoubling to obtain optical frequencies from frequency standardsoperating in lower frequency ranges. In this context, opticalfrequencies are infrared, visible and ultraviolet. The low-frequencylimit of the infrared corresponds to a wavelength of about 100 microns.

SUMMARY OF THE INVENTION According to my invention, l have recognizedthat optical stimulated emission structures can be adapted for use inoptical frequency standards by disposing within an optical resonator anactive medium and an optical power limiter providing a power-dependentbut frequency-independent loss approaching the maximum gain of theactive medium with respect to frequency. Accordingly, the limiter isadapted to providea frequency-independent loss that approaches the peakgain of the medium at a power level below that at which the gain beginsto saturate. If the gain is unsaturated, the gainversus-frequencycharacteristic is peaked at the atomic line center. If these conditionsare satisfied, the linewidth of the oscillator will be /21, where c isthe velocity of light and Lis the total optical length of the resonator.In the presence of a mechanical disturbance of the resonator, themaximum frequency excursion from the center of the atomic gain curvewill be limited to c/4L in either direction. Since'the effectivelinewidth of the laser oscillation is c/ZL it can be made arbitrarilysmall by making L correspondingly large.

The limiteradvantageously provides the laser with a selfstabilizingcharacteristic that minimizes the effects of random fluctuations in the'gain of the active medium. If the gain suddenly changes, the loss inthe limiter also changes in the same sense so that it limits atessentially the same level as before.

According to one feature of my invention, the optical resonator is madesufficiently long to have a resonance within a narrow effectivelinewidth by, for example, including therein an optical delay line suchas a multiple-pass reflective structure, sometimes called a foldedoptical delay line.

According to another feature of my invention, the optical power limitermay comprise a negative thermal lens in tandem with at least oneabsorbing body or other lossy element that has an aperture disposed topass the central portion of the diverging beam. As disclosed in thecopending application of TC. Damen et a1., Ser. No. 525,216, filed Feb.4, 1966 and assigned to the assignee hereof, now U.S. Pat. No.3,434,779, the negative thermal lens may be a liquid thermal lens inwhich a radiation-absorbing liquid produces a defocusing refractiveindex gradient transverse to the path of the radiation therein. Theamount of absorption and the amount of defocusing is'directly related tothe power of the radiation passing through the liquid. Nevertheless,unlike the arrangement shown in the above-cited application of Damen etal.,. the limiter employed in a laser according to my invention isadapted to provide a loss approaching the unsaturated peak gain of theactive medium.

BRIEF DESCRIPTION OF THE DRAWING Further features and advantages of myinvention will become apparent from the following detailed description,taken together with the drawing, in which the sole FIG. is a partiallypictorial and partially schematic illustration of a preferred embodimentof the invention.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENT In the drawing, there is shown alaser that is suitable for use as an optical frequency standard. Thelaser illustratively comprises a gaseous active medium contained in aglass envelope 11 that has Brewster-angle end windows 12 and 13 andlaterally disposed regions in which the anode 14 and the cathode 15 arecontained. Electrical excitation power is applied to the gaseous activemedium through electrodes 14 and 15 by connecting the anode 14 to thepositive terminal of voltage source 16 and the cathode 15 to thenegative terminal of source 16. The electrical excitation power is saidto pump the active medium.

The envelope 11 and the active medium which it contains are disposed inan optical resonator formed by the focusing reflectors l7 and 18.Reflector 1 is coated to be opaque; but reflector 18 is coated to bepartially transmissive, so that output radiation is obtained at thedesired frequency. This output radiation can be used to provide afrequency against which other frequencies can be compared, or for anyother use of a highly stable, monochromatic optical frequency.

Also included between end window 13 and output reflector 18, are a powerlimiter 19 and an optical delay line 20.

The power limiter l9 illustratively includes a liquid thermal lens 24,which is a negative lens in which the strength of the defocusing efiectis thermally responsive, i.e., directly related to the power ofradiation absorbed in the liquid. The thermally responsive liquid inlens 24 is illustratively nitrobenzene. The lens 24 includes aglass-walled container with Brewster-angle end windows, which transmitthe radiation not absorbed by the liquid in the container. The powerlimiter 19 also includes the apertured members 25 and 26, illustrativelyblack absorbing cardboard or absorbing glass, disposed on opposite sidesof the lens 24 with their apertures centered in the path of theradiation. The area of each aperture is selected to be sufficiently lessthan the normal cross-sectional area of the beam that would exist in theabsence of power limiter 19, so that the gain of the laser isessentially unsaturated at the radiation level at which the laseroscillates. It should be noted that the members 25 and 26 can beeliminated if the tube 11 has a sufficiently small bore and one ofreflectors 22 or 23 has a sufficiently small aperture to be theequivalent apertured means for passing only a limited central portion ofthe defocused radiation.

The optical delay line illustratively includes a reflective,multiple-pass structure that provides a desired optical path length inthe laser without imposing any additional resonator modes upon thelaser. The path length is sufficient to make the overall mode frequencyspacing of the laser equal to or less than the narrow effectivelinewidth, as affected by limiter 19. The mode frequency spacing is thefrequency difference between the center frequencies of adjacent resonantmodes of the complete laser. Specifically, the delay line 20 comprisesthe nearly concentric reflectors 22 and 23 having apertures positionedso that the beam enters oblique to a radius of reflector 22 andcontinues to be reflected in directions oblique to local radii of eitherreflectors 22 and 23 until the beam exits through the aperture ofreflector 23. Preferably, the reflectors 22 and 23 are made slightlyastigmatic, for example, by increasing or decreasing the radius ofcurvature in one coordinate but not the other. The number of multiplepasses can be made very great by this method without permitting anyportion of the beam to have a reentrant path within delay line 20.

A reentrant path is a path in which a portion of the beam can retraceits path, or resonate, within delay line 20 alone. The lack of anyreentrant path within delay line 20 prevents it from imposing its ownresonant modes upon the resonant modes of the complete resonator of thelaser.

Typically, reflectors 22 and 23 would have broadband reflectivity,although this characteristic is not required if they have their peakreflectivity in a frequency range encompassing the narrow effectivelinewidth of the complete laser.

In operation, it is not necessary to perform a vemier, or fine,adjustment of the overall resonator length to bring a resonatorresonance close to the atomic line center frequency of the activemedium, as the atomic line center frequency is also essentially thecenter frequency of the narrow effective linewidth of the completelaser. The laser will always oscillate only on that cavity mode which isclosest to the atomic line center. This mode should cause the limiter tolimit at a level for which all other cavity modes will be suppressed. Asthe length of the resonator is changed, another cavity mode may finditself closest to line center, and power will build up in this mode,suppressing the first one. Thus, the maximum frequency excursion of ourfrequency standard from line center is one-half of the cavity modespacing, or c/4L(c being the velocity of light and Lthe length of thecavity). This can be made small by making Lsufficiently long.

Suppose now that a transient occurs in the excitation power supplied bysource 16 to the gaseous medium in tube 11. The coherent light intensityin tube 11 will increase. Power limiter l9 absorbs some of theadditional light intensity in the liquid of lens 24. The thermalgradient changes because the liquid increases in temperature more on theaxis of the beam than near the edges of the beam. Correspondingly, theindex of refraction changes (decreases) more on the axis than near theedges of the beam. ln nitrobenzene and most other liquids in whichabsorption changes optical properties, the resulting index of refractiongradient produces increased defocusing of the beam. The apertures inmembers 25 and 26 pass a smaller portion of the beam, in terms ofcross-sectional area. The result is that the intensity of the portion ofthe beam which is passed remains essentially constant.

The foregoing sequence of events can be summarized in the followingterms of the optical art. The absorbing liquid acts like a negative lenswhen a laser beam passes through it; and the power of this lens changesin direct relation to the changing intensity of the laser beam passingthrough it. Thus, for increasing laser intensities, the aperture willintercept a larger portion of the incident beam power and will produceincreased loss in such an amount that the transmitted portion of thebeam remains almost constant. Similarly, if the light intensity in tube11 decreases, the power of the negative lens 24 decreases, and the losspresented to the beam decreases by substantially the amount expected inlimiter.

The limiting level of limiter 19 can be adjusted by varying theabsorption of the liquid, the size of the apertures, or the single-passpath length of the light within the liquid.

It should he understood that other absorption phenomena could beemployed to implement a limiter suitable for use in my invention. Forexample, it is known a parametric oscillator has a threshold pumpingpower. In one modifica ion of the embodiment of the drawing, limiter 19includes, instead of the components shown, a nonlinear crystal suitablefor a parametric oscillator. The coherent light in the laser is employedas the pumping power to generate a parametric oscillation in thecrystal. The parametrically generated radiation then propagates out ofthe crystal without entering any other components of the laser. Theunabsorbed, threshold pumping light is all that remains to propagateinto delay line 20. The unabsorbed light is limited at the thresholdlevel. Raman or Brillouin scattering may be employed to eliminate theunwanted coherent light power in a similar fashion, provided thescattering occurs laterally and not appreciably in the forwarddirection.

Moreover, other techniques exist for providing the desired optical pathlength in the resonator. For example, a long laser resonator might havesufficient path length to permit elimination of delay line 20, althoughthe resulting apparatus would be larger than usually desirable. Otherforms of optical delay line might include prisms of relatively highindex of refraction. Such prisms would be adapted for multiple internalreflection of the beam in a nonreentrant path. Optical path length isdirectly proportional to index of refraction.

My invention is also applicable to ring lasers.

lclaim:

l. A laser comprising first and second reflectors forming a resonator,an active laser medium disposed within said resonator, means for pumpingsaid active medium, and an optical power limiter disposed within saidresonator for providing a loss approaching the peak gain of said activemedium with respect to frequency for a power level below that at whichthe gain of said medium would begin to saturate.

2. A laser according to claim 1 in which the resonator includes anoptical delay line of length making the overall longitudinal modefrequency spacing of the laser equal to or less than the effectivelinewidth of said laser, as affected by the limiter.

3. A laser comprising an optical resonator of path length I, an activelaser medium disposed within said resonator, means for pumping saidactive medium, and means for limiting the power level within saidresonator with a loss substantially equal to the unsaturated peak gainof said active medium with respect to frequency, whereby the laser hasan effective linewidth suitable for a source of standard frequency, saidpath length Lbeing selected to make the overall longitudinal modefrequency spacing of the laser equal to or less than said effectivelinewidth of said laser.

4. A laser according to claim 3 in which the resonator includes anonreentrant folded optical delay line providing a major portion of thepath length L

1. A laser comprising first and second reflectors forming a resonator,an active laser medium disposed within said resonator, means for pumpingsaid active medium, and an optical power limiter disposed within saidresonator for providing a loss approaching the peak gain of said activemedium with respect to frequency for a power level below that at whichthe gain of said medium would begin to saturate.
 2. A laseR according toclaim 1 in which the resonator includes an optical delay line of lengthmaking the overall longitudinal mode frequency spacing of the laserequal to or less than the effective linewidth of said laser, as affectedby the limiter.
 3. A laser comprising an optical resonator of pathlength L, an active laser medium disposed within said resonator, meansfor pumping said active medium, and means for limiting the power levelwithin said resonator with a loss substantially equal to the unsaturatedpeak gain of said active medium with respect to frequency, whereby thelaser has an effective linewidth suitable for a source of standardfrequency, said path length L being selected to make the overalllongitudinal mode frequency spacing of the laser equal to or less thansaid effective linewidth of said laser.
 4. A laser according to claim 3in which the resonator includes a nonreentrant folded optical delay lineproviding a major portion of the path length L.